1. Field of the Invention
This invention relates to the system and method for protecting microelectronic circuitry from damage that can be caused by electrostatic discharge therethrough while being placed in electrical interconnection in an operating system. More particularly, it relates to a system for predischarging electrostatic charge that may be present on printed circuit assemblies prior to insertion in an associated connector, thereby protecting Electrostatic Sensitive Devices that may be mounted thereon from damage or destruction.
2. State of the Prior Art
It has been recognized that with the advent of microcircuitry, including the use of integrated circuitry, an ever-increasing problem of damage or destruction has resulted from inadverdent electrostatic discharge therethrough. These Electrostatic Sensitive Devices (ESD) includes circuits incorporating metal oxide semiconductors, field effect transistors, complementary metal oxide semiconductors, and the like. Electrostatic Discharge damage has been detected in many logic family arrangements, and in particular, considerable testing and evaluation has gone forward regarding the 10K ECL logic family. Problems are known to exist in gate protected MOSFETS, input protected MOS Dynamic RAMS and TTL Logic Gates. The Advancement of integrated circuitry technology has resulted in many more devices being susceptible to electrostatic discharge damage because higher densities and smaller sizes results in less physical isolation between metalizations and thinner oxides separating diffusion layers. It is common for the ESD circuits to be mounted on printed circuit (PC) boards for making electrical interconnections through connectors mounted in a back panel assembly in the electronic device.
The nature of the damage or destruction to the ESD resulting from electrostatic discharge therethrough has been studied and reported in various technical journals. Suffice it to say that there can be damage or destruction of the input circuit, or the output circuit, depending upon the electrostatic discharge path. The undesired electrostatic discharge through the ESD can occur in several ways. For example, if a terminal of the ESD is inadvertently touched by an undischarged human, there is normally sufficient electrostatic charge on the body to damage or destroy the circuit. It is not uncommon for an electrostatic charge of 2,000 volts to be on a human, the discharge of which is normally below the feeling of sensation to the person, but is more than adequate to destroy an ESD circuit. Under proper conditions the charge on the human body can raise to the level of 5,000 to 6,000 volts, which is normally at the level when a "shock" is felt when it is discharged. The electrostatic discharge can also occur when the ESD is brought into contact with some other object that is in a condition to cause electrostatic discharge. For example being placed on a work table that is not properly protected can cause the electrostatic discharge to the ESD. Further, inserting the PC board into a back panel without protection for the electrostatic discharge can cause damage or destruction of the ESD.
ESD failure analysis indicates that 40 to 50% of the failures occur as a result of uncontrolled electrostatic discharge through the ESD. Damage can occur during manufacturing process, during handling while shipping and moving, and upon insertion into the device with which the ESD is to function. Fixed electrostatic-free work stations have been utilized for a considerable period of time, and involve the use of conductive table tops that are provided with the suitable grounding system, in conjunction with a grounding system for the personnel. The grounding system for the personnel at the work station often involves the use of a conductive wrist strap for draining away electrostatic charge. It is also common to utilize a conductor impregnated floor in conjunction with metallic elements worn on the feet of personnel for draining away the electrostatic charge. During shipping, it has been advantageous to protect the ESD circuits by placing them in bags constructed of electrostatic protective materials of which there are several varieties providing a variety of protection levels. For shipping, containers having conductive carbon black particles coated thereon for shielding against electrostatic charge from outside the container to the ESD circuits therein is described in U.S. Pat. No. 4,293,070. Finally, there is the damage or destruction that can be caused during insertion of the PC board in the cabinet that contains the ESD circuits. Characteristically, protection has been attempted to be afforded by personnel again wearing conductive grounded wrist straps to drain away electrostatic charge from the person. It has been determined that if the person is not grounded, a charge can be transferred to the assembly. Sliding the PC board in guide channels that are often constructed of a plastic or nylon material does not discharge electrostatic charge and damage to the ESD is caused when a connector pin makes contact and discharges the electrostatic charge.
The conductive wrist strap provided an impediment to free movement, and in a field condition may be resisted by personnel involved with maintenance and operation of the system. Once recognized, the problem of providing electrostatic free shipping containers and controlled work stations for manufacturer and repair can be quite readily provided, controlled, and maintained. However, the problem of field insertion of the PC board with its attendant electrostatic discharge problem is the least easy to subject to on-going control.